For the purpose of removing substances usually eliminated with urine and for the purpose of withdrawing fluid, use is made of various methods for extracorporeal blood treatment and cleaning. In the case of hemodialysis, the patient's blood is cleaned outside the body in a dialyzer. The dialyzer comprises a blood chamber and a dialyzing fluid chamber, which are separated by a semipermeable membrane. During the treatment, the patient's blood flows through the blood chamber. In order to clean the blood effectively from substances usually eliminated with urine, fresh dialyzing fluid continuously flows through the dialyzing fluid chamber.
Whereas the transport of the smaller molecular substances through the membrane of the dialyzer is essentially determined in the case of hemodialysis (HD) by the concentration differences (diffusion) between the dialyzing fluid and the blood, substances dissolved in the plasma water, in particular higher molecular substances, are effectively removed in the case of the hemofiltration (HF) by a high fluid flow (convection) through the membrane of the dialyzer. The dialyzer acts as a filter in hemofiltration. Hemodiafiltration (HDF) is a combination of the two methods.
In the case of hemo(dia)filtration, a part of the fluid withdrawn from the blood through the membrane of the dialyzer is replaced by a sterile substitution fluid, which is generally fed to the extracorporeal blood circuit either upstream of the dialyzer or downstream of the dialyzer. The supply of the substitution fluid upstream of the dialyzer is also referred to as predilution and the supply downstream of the dialyzer as postdilution.
Apparatuses for hemo(dia)filtration are known in which the dialyzing fluid is prepared online from fresh water and dialyzing fluid concentrate and the substitution fluid is prepared online from the dialyzing fluid.
In the case of the known hemo(dia)filtration apparatuses, the substitution fluid (substituate) is fed to the extracorporeal blood circuit from the fluid system of the machine via a substituate supply line. In the case of predilution, the substituate line leads to a junction point on the arterial blood line upstream of the dialyzer or filter, whereas in the case of postdilution the substituate line leads to a junction point on the venous blood line downstream of the dialyzer or filter. The substituate line comprises for example a connector, with which it can be connected either to the venous or arterial blood line. In order to interrupt the fluid supply, a clamp or the like is provided on the substituate line. Such a hemo(dia)filtration apparatus is known for example from EP-A-0 189 561.
In the known methods of chronic blood-cleaning therapy, for example hemodialysis, hemofiltration and hemodiafiltration, an arteriovenous fistula is often applied surgically as an access to the blood vessel system. The use of an implant is also possible. When mention is made below of a “fistula”, this is understood to mean any kind of connection between a vein and an artery of the patient.
The blood flowing through the fistula is used only during the actual dialysis treatment. In the period free from dialysis, the blood flow in the fistula corresponds to a functional left/right shunt, wherein a part of the arterial blood is fed from the heart minute volume (HMV), bypassing a peripheral use, directly to the venous system and the heart. The fistula flow recirculates via the heart and lungs. The functional part of the fistula flow in the heart minute volume is defined as the cardiopulmonary recirculation.
The cardiopulmonary recirculation not only has effects on the patient's circulatory load, but also on the efficiency of the dialysis. Since the dialyzed blood from the extracorporeal circuit is mixed with the venous backflow from the large body circulation thereby bypassing the systemic circulatory areas, a systematic reduction in the concentration of the dialyzable constituents in the arterial blood results (D. Schneditz et al.: Cardiopulmonary recirculation during hemodialysis. Kidney Int. 42: 1450-1456, 1992).
Of importance for the functional capability of fistulas is their perfusion. If the fistula flow falls below a critical value, the risk of a fistula thrombosis then increases with the possible loss of the vascular access, which in dialysis treatment represents a considerable complication (W. Bay et al.: Color Doppler flow predicts PTFE graft failure, J. Am. Soc. Nephrol. 5: 407 (1994)). If the fistula flow during the dialysis treatment is smaller than the extracorporeal blood flow(QB), local fistula recirculation occurs, whereby a fraction of the dialysed blood fed back to the fistula with the venous blood line is again fed to the dialyzer via the arterial blood line. Fistula recirculation RA causes a significant reduction in the dialysis efficiency (F. Gotch: “Models to predict recirculation and its effects on treatment time in single-needle-dialysis”, First Intl. Symposium on Single-Needle-Dialysis, publisher: S. Rignoir, R. Vanholder and P. Ivanovich, Cleveland, ISAO Press, 1984, page 305 ff.). The measurement of the quality of the vascular access is therefore an important means of quality assurance in dialysis treatment.
On account of its clinical importance, various methods are known for measuring the fistula recirculation (RA). Common to all of them is the measurement of a physical or chemical characteristic quantity of the blood, which is changed in the venous branch of the extracorporeal circuit. The physical or chemical characteristic quantity of the blood can be changed by a manual injection of an indicator or also indirectly via the dialyzer preparation unit.
A method for the measurement of recirculation referred to as thermodilution is known from EDTNA-ERCA Journal 19, 6 (1993). With the known method, a brief drop in temperature is initiated in the dialyzing fluid circuit, which is transferred to the venous branch of the extracorporeal circuit and leads to a detectable temperature jump in the arterial branch of the extracorporeal circuit when a recirculation occurs.
A known device for performing the method referred to as thermodilution comprises a temperature sensor arranged in the arterial branch and one in the venous branch of the extracorporeal circuit. The venous temperature sensor is used to detect the temperature jump that is attributable to the drop in temperature produced in the dialyzing fluid circuit. The measured temperature jump is integrated over time or otherwise characterized and is subsequently compared with the temperature course recorded in the arterial measuring sensor. The ratio of the two temperature integrals or other characteristic quantities with respect to one another is a measure of the overall reduction in efficiency of the dialysis treatment due to fistula and cardiopulmonary recirculation.
The known device for the measurement of recirculation has been tried and tested in practice. It proves to be a drawback, however, that only the total recirculation denoted below as recirculation R can be measured, which corresponds to the sum of fistula recirculation RA and cardiopulmonary recirculation RCP.
A method for measuring recirculation R, i.e. the sum of the fistula recirculation (RA) and the cardiopulmonary recirculation (RCP), is also known from DE 197 02 441 C1. With the known method, a physical or chemical characteristic quantity of the dialyzing fluid is changed in the dialyzing fluid path upstream of the dialyzer, which leads to a change in the physical or chemical characteristic quantity on the blood side. The change in the characteristic quantity of the dialyzing fluid on the blood side leads to a change in the characteristic quantity of the dialyzing fluid downstream of the dialyzing fluid chamber of the dialyzer. In order to determine the recirculation, the characteristic quantity is measured in the dialyzing fluid path downstream of the dialyzer and recirculation R is determined from the course of the change in the characteristic quantity as a function of time. As the physical or chemical characteristic quantity, the ion concentration of the dialyzing fluid, for example the Na concentration of the dialyzing fluid, or also the temperature of the dialyzing fluid can be changed and measured. The drawback, however, is once again that with the known method it is not possible to distinguish between fistula recirculation RA and cardiopulmonary recirculation RCP.
DE-A-195 28 907 C1 describes a method for determining the cardiopulmonary recirculation. The measurement of the cardiopulmonary recirculation is based on two measurements of the recirculation fraction following one another in close succession, which are carried out automatically before and after the reversal of the blood flow. The drawback is that the known method requires the reversal of the blood flow.
A method for determining the recirculation is known from U.S. Pat. No. 6,537,240 B2, which is based on a change in the composition of the blood in the extracorporeal blood circuit due to an increase or reduction in the ultrafiltration rate within a predetermined time interval.